1. Field of the Invention
The present invention relates to a valve mounting structure for a fuel tank, in which a float valve with floats accommodated in a float case and lifted and lowered in response to the raising and lowering of the level of fuel is mounted to an upper wall of a tank body.
2. Description of the Related Art
There is a structure for mounting a float valve to an upper wall of a tank body of a fuel tank for an automobile, which is described in Japanese Patent Application Laid-open No.7-293382, and in which the float valve is inserted into the tank body through an opening defined in the upper wall of the tank body, and a flange formed on an outer peripheral surface of the float valve is placed into abutment against an outer periphery of the opening with a seal member interposed therebetween, and is bolted to the outer periphery of the opening.
There is also a known structure for mounting a float valve to a fuel tank having a tank body made of a synthetic resin, in which the float valve made of a synthetic resin is fitted to an inner periphery of a cover made of a synthetic resin and coupled to the inner periphery of the cover by a concavo-convex engagement, and is inserted through an opening in an upper wall of the tank body, and the cover is fixed to an outer periphery of the opening by welding.
The tank body in the latter structure is formed of a synthetic resin having a rate of expansion caused by contact with fuel (which will be referred to as a swelling rate hereinafter) greater than that of a body portion of the float valve, and the cover welded to the tank body is also formed of the same material as the tank body in order to ensure a welding strength. Therefore, the latter structure has a problem in that when the cover is swelled by the contact with the fuel and, as a result, the concavo-convex engagement of the cover with the float valve is loosened, the float valve is liable to be dropped out from the cover.
The present invention has been developed with the above circumstance in view, and it is an object of the present invention to reliably mount a float valve to a tank body made of a synthetic resin by welding.
To achieve the above object, the present invention provides a valve mounting structure of a fuel tank, in which a float valve with floats accommodated in a float case and lifted and lowered in response to raising and lowering of the level of a fuel is mounted to an upper wall of a tank body. A welding flange is provided on a cover made of a synthetic resin is welded to an outer periphery of an opening defined in the upper wall of the tank body made of a synthetic resin so that the cover covers the opening, and a fixing flange provided at an upper portion of a protector for supporting the float case of the float valve by a concavo-convex engagement is fixed between the outer periphery of the opening of the tank body and the welding flange of the cover, the protector having a swelling rate less than those of the tank body and the cover.
With the above arrangement, each of the tank body and the cover is formed of the synthetic resin having a larger swelling rate. Therefore, the tank body and the cover welded to each other swell at the same rate, whereby a load is prevented from being applied to a weld zone. Moreover, because the protector is formed of the material having the swelling rate less than those of the tank body and the cover and, hence, it is possible to reliably prevent the float case from dropping out due to the swelling of the protector. In addition, the protector is fixed with its mounting flange sandwiched between the tank body and the cover, rather than being welded directly to the tank body and the cover. Therefore, even if the swelling rate of the tank body and the cover is different from that of the protector, the protector can be fixed firmly, while effectively absorbing a difference between the swelling rates.